Preheat the oven to 375°F (190°C). Line a baking sheet or 9x13-inch dish with parchment for easy cleanup.
Prep the peppers: Slice each bell pepper in half lengthwise and remove seeds and membranes.
Place cut-side up on the baking sheet. Sprinkle with a little salt and pepper.
Soften the peppers (optional for extra tenderness): Bake the empty pepper halves for 10 minutes while you make the filling.
Make the filling: In a large bowl, mix shredded chicken, Buffalo sauce, cream cheese, sour cream, melted butter, garlic powder, onion powder, 1/2 cup mozzarella, and cheddar. Season with salt and pepper.
Fill the peppers: Spoon the Buffalo chicken mixture evenly into each pepper half.
Top with the remaining 1/2 cup mozzarella.
Bake for 18–22 minutes, until the cheese is melted and the peppers are tender but not mushy. If you like a browned top, broil for 1–2 minutes at the end.
Finish and serve: Let cool for 5 minutes. Garnish with green onions and a light drizzle of ranch or blue cheese dressing, if desired.